Thesis: Ph. D., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2018. / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 259-268). / This work investigates how a Lagrangian perspective applies to models of two oceanographic flows: an overturning submesoscale eddy and the Western Alboran Gyre. In the first case, I focus on the importance of diffusion as compared to chaotic advection for tracers in this system. Three methods are used to quantify the relative contributions: scaling arguments including a Lagrangian Batchelor scale, statistical analysis of ensembles of trajectories, and Nakamura effective diffusivity from numerical simulations of dye release. Through these complementary methods, I find that chaotic advection dominates over turbulent diffusion in the widest chaotic regions, which always occur near the center and outer rim of the cylinder and sometimes occur in interior regions for Ekman numbers near 0.01. In thin chaotic regions, diffusion is at least as important as chaotic advection. From this analysis, it is clear that identified Lagrangian coherent structures will be barriers to transport for long times if they are much larger than the Batchelor scale. The second case is a model of the Western Alboran Gyre with realistic forcing and bathymetry. I examine its transport properties from both an Eulerian and Lagrangian perspective. I find that advection is most often the dominant term in Eulerian budgets for volume, salt, and heat in the gyre, with diffusion and surface fluxes playing a smaller role. In the vorticity budget, advection is as large as the effects of wind and viscous diffusion, but not dominant. For the Lagrangian analysis, I construct a moving gyre boundary from segments of the stable and unstable manifolds emanating from two persistent hyperbolic trajectories on the coast at the eastern and western extent of the gyre. These manifolds are computed on several isopycnals and stacked vertically to construct a three-dimensional Lagrangian gyre boundary. The regions these manifolds cover is the stirring region, where there is a path for water to reach the gyre. On timescales of days to weeks, water from the Atlantic Jet and the northern coast can enter the outer parts of the gyre, but there is a core region in the interior that is separate. Using a gate, I calculate the continuous advective transport across the Lagrangian boundary in three dimensions for the first time. A Lagrangian volume budget is calculated, and challenges in its closure are described. Lagrangian and Eulerian advective transports are found to be of similar magnitudes. / by Genevieve Elizabeth Brett. / Ph. D.

Thesis: S.M., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2016 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 119-123). / Most traditional theories regarding upper oceanic response due to passing tropical cyclones involve an initial and predominant upwelling based on storms' strong cyclonic flow and resulting positive wind stress curl imparted on the sea surface. In August 2015, Air-Launched Autonomous Micro Observer float 9077 was intercepted by Hurricane Ignacio and its temperature measurements revealed a 40 m depression of the 26°C isotherm ahead of the device's closest point of approach with the storm and usual upwelling response. This unique finding motivated attempting to replicate the apparent downwelling ahead of Hurricane Ignacio and three others-Irma, Florence and Michael, using the Price-Weller-Pinkel 1/2 order closure model, via comparisons of the depth of the 26°C isotherm, tropical cyclone heat potential and vertical velocity. When modeling the total stress, two other traditional ideas were challenged. / First, many legacy drag coefficients linearly increase with wind speed, while the modem variety maximize near tropical cyclone strength, with varying asymptotic and/or decreasing end behavior. Second, it is believed that sea spray droplets are sheared off the largest wave crests, quickly accelerate in the high winds, but upon reentry, dampen the smaller waves and flux substantial amounts of momentum to the sea. Taken together, many traditional atmosphere-ocean models bulk parameterize air-sea interaction processes and employ a legacy drag coefficient and omit or crudely formulate sea spray. Therefore, this study aimed to simulate the aforementioned downwelling using 14 total forcing parameterizations, including seven different legacy or modem drag coefficients, with and without spray stress. / A combination of qualitative and statistical analyses illustrated downwelling was present in Hurricane Ignacio and every other storm by a large majority of variable indices, legacy drag coefficients were statistically significantly over-estimating outliers and should not be employed in tropical cyclone models, and while the explicit addition of sea spray to interfacial stress reduced model accuracy, this phenomenon remains paramount through modem drag coefficient selection. The confirmation of downwelling is physically founded in Ekman dynamics and may be significant in storm surge enhancement due to the accompanying surface height anomaly and near-shore depth limitations forcing water, with a negative vertical velocity, ashore. / Office of Naval ResearchN00014-15-12293 / Office of Naval ResearcN00014-18-12819 / U. S. Navy Civilian Institution Program / by Stephan D. Gallagher. / S.M. / S.M. Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)

Thesis: S.M., Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2020 / Cataloged from student-submitted PDF of thesis. / Includes bibliographical references (pages 91-94). / The Atlantic Water (AW) Layer in the Arctic Subpolar gyre sTate Estimate (ASTE), a regional, medium-resolution coupled ocean-sea ice state estimate, is analyzed for the first time using bounding isopycnals. A surge of AW, marked by rapid increases in mean AW Layer potential temperature and AW Layer thickness, begins two years into the state estimate (2004) and traverses the Arctic Ocean along boundary current pathways at approximately 2 cm/s. The surge also alters AW flow direction and speed including a significant reversal in flow direction along the Lomonosov Ridge. The surge results in a new quasi-steady AW flow from 2010 through the end of the state estimate period in 2017. The time-mean AW circulation during this time period indicates a significant amount of AW spreads over the Lomonosov Ridge rather than directly returning along the ridge to Fram Strait. A three-layer depiction of ASTE's overturning circulation within the AO indicates AW is converted to colder, fresher Surface Layer water at a faster rate than is transformed to Bottom Water (1.2 Sv vs. 0.4 Sv). Observed AW properties compared to ASTE output indicate increasing misfit during the simulated period with ASTE's AW Layer generally being warmer and thicker than in observations. / by Jeffrey Scott Grabon. / S.M. / S.M. Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Science; and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (p. 134-138). / A numerical model of the tropical Atlantic ocean is used to investigate the upper layer pathways of the Meridional Overturning Circulation (MOC) in the tropical Atlantic. The main focus of this thesis is on those parts of the tropical circulation that are thought to be important for the MOC return flow, but whose dynamics have not been understood yet. It is shown how the particular structure of the tropical gyre and the MOC act to inhibit the flow of North Atlantic water into the equatorial thermocline. As a result, the upper layers of the tropical Atlantic are mainly fed by water from the South Atlantic. The processes that carry the South Atlantic water across the tropical Atlantic into the North Atlantic as part of the MOC are described here, and three processes that were hitherto not understood are explained as follows: The North Brazil Current rings are created as the result of the reflection of Rossby waves at the South American coast. These Rossby waves are generated by the barotropically unstable North Equatorial Countercurrent. The deep structure of the rings can be explained by merger of the wave's anticyclones with the deeper intermediate eddies that are generated as the intermediate western boundary current crosses the equator. The bands of strong zonal velocity in intermediate depths along the equator have hitherto been explained as intermediate currents. Here, an alternative interpretation of the observations is offered: The Eulerian mean flow along the equator is negligible and the observations are the signature of strong seasonal Rossby waves. The previous interpretation of the observations can then be explained as aliasing of the tropical wave field. / (cont.) The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability waves. The equatorial current system with its strong shears is unstable and generates tropical instability waves. These waves cause a poleward temperature flux which steepens the isotherms which in turn generates are geostrophically balanced zonal flow. In the eastern part of the basin this zonal flow feeds the southeastward flow of the equatorial gyre. / by Markus Jochum. / Ph.D.

Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009. / Includes bibliographical references (leaves 49-51). / If the ocean is in a statistically steady state, energy balance is a strong constraint, suggesting that the energy input into the world ocean is dissipated simultaneously at the same rate. Energy conservation is one of the most important principles in the natural world. However, the study of energy balance in the oceanic circulation has long been overlooked. Mink and Winch (1998) proposed that energy is needed to maintain the meridional overturning circulation and they also concluded that the wind energy input into the world ocean constitute the most important part. Since then, many estimates on the wind energy input have been given with a focus on different time and spatial scales. It is well known that it is the air-sea momentum flux (wind stress) that actually drives the ocean circulation, especially the upper layer circulation. Due to the difficulties of directly measuring the wind stress, different algorithms were proposed to relate the wind stress with the wind velocity and other related atmospheric and oceanic variables. Different algorithms in fact produce quite different wind stresses, which may leads to spurious estimates in the wind energy input into the world ocean. The thesis is organized as follows. In chapter 1, we try to understand the difference of four bulk algorithms, and conclude that different bulk algorithms may yield the wind energy input differences of 20%. Comparison of 4 different wind stress dataset were presented in Chapter 2. However, we do not determine which product is the best. In Chapter 3, a simple numerical experiment was executed and some preliminary estimate on the effects of introducing the wind stress dependence on the oceanic surface velocity were given. The ECCO data computation, however, does not produce the results as expected and some explanations are given. / by Shaoyu Yuan. / S.M.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (p. 207-211). / My thesis covers two general circulation problems that involve the stability of largescale oceanic flows and the importance of non-local effects. The first problem examines the stability of meridional boundary currents, which are found on both sides of most ocean basins because of the presence of continents. A linear stability analysis of a meridional boundary current on the beta-plane is performed using a quasi-geostrophic model in order to determine the existence of radiating instabilities, a type of instability that propagates energy away from its origin region by exciting Rossby waves and can thus act as a source of eddy energy for the ocean interior. It is found that radiating instabilities are commonly found in both eastern and western boundary currents. However, there are some significant dierences that make eastern boundary currents more interesting from a radiation point of view. They possess a larger number of radiating modes, characterized by horizontal wavenumbers which would make them appear like zonal jets as they propagate into the ocean interior. The second problem examines the circulation in a nonlinear thermally-forced twolayer quasi-geostrophic ocean. The only driving force for the circulation in the model is a cross-isopycnal flux parameterized as interface relaxation. This forcing is similar to the radiative damping used commonly in atmospheric models, except that it is applied to the ocean circulation in a closed basin and is meant to represent the large-scale thermal forcing acting on the oceans. / (cont.) It is found that in the strongly nonlinear regime a substantial, not directly thermally-driven barotropic circulation is generated. Its variability in the limit of weak bottom drag is dominated by highfrequency barotropic basin modes. It is demonstrated that the excitation of basin normal modes has significant consequences for the mean state of the system and its variability, conclusions that are likely to apply for any other system whose variability is dominated by basin modes, no matter the forcing. A linear stability analysis performed on a wind- and a thermally-forced double-gyre circulation reveals that under certain conditions the basin modes can arise from local instabilities of the flow. / by Hristina G. Hristova. / Ph.D.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1998. / Includes bibliographical references (p. 159-166). / by Daniel Reed Goldner. / Ph.D.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2009. / Includes bibliographical references (p. 165-168). / Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends 1 km above the topography suggesting that mixing is supported by breaking of gravity waves radiated from the ocean bottom. In other regions, gravity wave radiation by bottom topography has been primarily associated with the barotropic tide. In this study, we explore the alternative hypothesis that the enhanced mixing in the Southern Ocean is sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly-nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. A major finding is that the waves generated at the ocean bottom at finite inverse Froude numbers drive vigorous inertial oscillations. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations and differs substantially from the classical lee wave problem. The theoretical predictions are tested versus two-dimensional and three-dimensional high resolution numerical simulations with parameters representative of the Drake Passage region. Theory and fully nonlinear numerical simulations are used to estimate internal wave radiation from LADCP, CTD and topography data from two regions in the Southern Ocean: Drake Passage and the Southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation measured in the region. / by Maxim Nikurashin. / Ph.D.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 297-213). / This thesis explores the role that the circulation in the Gulf of Maine (GOM) plays in determining the distribution of dense aggregations of copepods. These aggregations are an important part of the marine ecosystem, especially for endangered North Atlantic right whales. Certain ocean processes may generate dense copepod aggregations, while others may destroy them; this thesis looks at how different characteristics of the GOM circulation fit into these two categories. The first part of the thesis investigates a hypothetical aggregation mechanism in which frontal circulation interacts with copepod behavior to generate a dense patch of copepods. The first two chapters of this thesis address this mechanism in the context of coastal river plumes and salinity fronts. One chapter describes the characteristics and variability of coastal freshwater and salinity fronts using a historical dataset and a realistic numerical model. The seasonal variability of freshwater is tied in part to seasonality in river discharge, while variability on shorter time scales in the frontal position is related to wind stress. Another chapter applies the hypothetical mechanism to idealized river plumes using a suite of numerical models. The structure of the plume and plume-relative circulation change the resulting copepod aggregation from what is expected from the hypothetical mechanism. The second part of the thesis discusses the GOM circulation and how it may eliminate copepod patches. The summertime mean surface circulation and eddy kinetic energy are computed from a Lagrangian drifter dataset and an adaptive technique that allows for higher spatial resolution while also keeping uncertainty low. Eddy diffusivity is also computed over different regions of the GOM in an attempt to quantify the spreading of a patch of copepods, and is found to be lower near the coast where right whales are often found feeding on copepod patches. In the next chapter, a numerical drifter dataset is used to understand how the results of the previous chapter depend upon the quantity of observations. It is found that the uncertainty in estimating eddy diffusivity is tightly coupled to the number of drifters in the calculation. / by Nicholas W. Woods / Ph.D.

Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2001. / Includes bibliographical references (p. 115-120). / The interaction between the eddy field and the mean flow near the Gulf Stream is studied here using satellite altimeter measurements and an eddy resolving numerical model. The eddy vorticity flux in the quasigeostrophic framework is obtained from the stream function standard deviation and spatial correlation function assuming the correlation function is homogeneous. An analytical expression is found for the stream function correlation using the altimetric and numerical data. Cases when the correlation function is anisotropic are compared to the isotropic case previously studied by Hogg (1993), who found that the eddy vorticity flux drives two counter rotating gyres on either side of the stream. The anisotropy can be important in the eddy vorticity flux, even when its departure from the isotropic case is small. Meridional or zonal anisotropies can drive recirculation gyres similar in strength and position to the ones driven by the isotropic case. The results when including anisotropy in the diagonal direction suggest that the homogenoeus assumption may not be valid. / by Juan Botella. / S.M.